AC voltage regulators are used to closely control and regulate the AC voltage level being delivered to a load connected to the output of the AC voltage regulator, regardless of the AC voltage variation at the input of the AC voltage regulator.
This has been traditionally done by various low frequency (LF), typically at 50 or 60 Hz, electrical mains magnetic structures. These structures are typically tapped at specific discrete transformer voltage taps in various transformers or transformer configurations. Nonetheless, all these structures rely on traditional AC switching devices such as relays or semiconductor devices such as silicon-controlled rectifiers (SCR)'s or gate turn off thyristor (GTO)'s connected as anti-parallel AC switches, TRIAC's, AC switches such as insulated-gate bipolar transistors (IGBT)'s, MOSFET transistors, and SCR's configured as AC switches, e.g. connected between rectifiers. These AC switches are selected and activated by the electronic control circuit to automatically switch the selected magnetic transformer structure tap, in turn adjusting the transformer or transformer configuration turns ratio to control the AC output voltage as close as possible to the desired level.
Another traditional method to regulate an output AC voltage is to use an electro-mechanically-adjusted auto-transformer that is driven by electrical mechanical means, such as a controlled electrical motor. The electronic control in this case senses the input voltage and then drives the electro-mechanical means to move the output contact to adjust the turns of the auto-transformer, in turn sets the correct turns ratio to fix the output AC voltage to the desired level. These electro-mechanically-adjusted auto-transformer devices are also LF magnetic structures, typically at 50 Hz or 60 Hz, and generally use carbon brushes to make the moving electrical contact to the auto-transformer windings. These brushes, however, undergo mechanical wear as such they need frequent maintenance and replacement.
A more sophisticated fully electronic version utilizes again LF mains transformers, typically at 50 Hz or 60 Hz, connected in series between the AC input and the AC output of the voltage regulator. As the input AC voltage level varies, the AC voltage regulator electronic control senses the input voltage level, and then sets up an in-phase positive or an in-phase negative differential AC voltage that adds or subtracts, to or from, the varying input AC voltage to maintain the output AC voltage to the desired set level. This traditional approach, in its various forms, still uses LF mains frequency transformers or LF magnetic structures, typically at 50 Hz or 60 Hz. In one configuration, the power electronics generates a LF mains frequency to correct the input AC voltage by a high frequency pulse width modulation (HF PWM) means, and this in-phase correction voltage to adjust the input AC mains voltage, is applied to the primary of the LF transformer, with the secondary of the LF transformer connected in series between the input and output of the AC power line. But still the magnetic structures used in these configurations, even though the power electronics operate at higher PWM frequencies, the final differential AC waveform is still applied to the LF series transformer, typically at 50 Hz or 60 Hz, hence the LF transformer or magnetic structures still have the disadvantage of size and weight.
A series AC voltage regulation method is disclosed in U.S. Pat. No. 5,747,972. This patent discloses a particular method of using only a simple voltage polarity control, which is a limited and simple method of control. It further describes the limit of the control switching states of power semiconductor devices that is created by only using the simple voltage polarity control method at the low AC input voltage positive and negative levels around the zero voltage crossover points. To solve this problem using only simple voltage polarity control method in this indeterminate low positive or negative input voltage level range around the zero voltage crossover points leads to an ambiguous determination of the actual input voltage polarity because of the low zero crossing AC input voltages, and hence the status of the PWM switching sequence of the power semiconductors at these low positive or negative voltage levels of the AC input voltage zero crossover points. The '972 patent discloses this input AC voltage polarity indeterminate low voltage level state at below 4 volts, positive and negative—13.65, 14.5 and again 17.65, 18.5, 18.10. Accordingly, an abnormal switching sequence is generated by turning on, for a short period (e.g. 13.65 microseconds as disclosed), all of the power semiconductor switching devices. This creates a power semiconductor “shoot-through” that short circuits the input AC power source, which can cause degrading or destructive damage of the semiconductor power devices. “Shoot-through” is a term very clearly understood in the electronics industry as a very serious condition that must be avoided for power semiconductors, and engineers are always critically concerned about “shoot-through” because of its degrading and destructive impact on the power semiconductors.
It is clearly taught in the '972 patent about the aforesaid problem but seemingly rely on the actual low AC input voltage at the points of positive and negative AC input voltage near zero crossover, and also the high voltage characteristics of power semiconductor conducting voltages. The '972 patent teaches to create a deliberate shoot-through PWM switching sequence status (13.65, 14.5, and 17.65, 18.5, 18.10). Thus, because of the limitation of using AC input voltage polarity control, the '972 patent attempts to solve this problem by actually generating a deliberate “shoot-through” state of the power semiconductor switching devices that actually short circuits the AC input voltage source. This is a critical compromise, and especially in the case with modern power semiconductor devices which have much lower on-resistance and depend on the low source resistance of the AC input power supply or circuit capacitance near the semiconductor power switches. Furthermore, a “shoot-through” can also create an undefined uncontrolled current steering in the power inductor that negatively impacts operating performance. Therefore, the series AC voltage regulation method as disclosed in U.S. Pat. No. 5,747,972 has serious shortcomings.